1. Field of the Invention
This invention relates to the field of simulcast transmission systems and specifically to a simulcast transmission system wherein a transmitted signal is used to distribute audio as well as provide a reference to the remote transmitters of a simulcast system.
2. Description of the Prior Art
Simulcast transmission systems have been developed as a method for obtaining wide broadcast coverage in a large area with a diverse topology. Typically, these systems incorporate several transmitters which are placed a great distance from each other as determined by the constraints of a particular area. To achieve good quality audio reproduction in a simulcast transmission system, the transmitters of the simulcast system must be substantially exactly the same frequency. Any variation in the signals being transmitted in the system will result in various types of distortion in the signal delivered to a receiver. This requires that every transmitter in a simulcast system produce a signal of substantially identical frequency and substantially identical modulation.
The frequency and modulation characteristics of a simulcast transmitter can be relaxed somewhat if the system is used to distribute data. Modern data transmission and error correction techniques allow data to be recovered even though it may be slightly distorted.
Certain simulcast systems in the past have used high stability quartz crystal oscillators to control the transmitters' operating frequency and microwave modems and microwave systems have been used to distribute the audio signal. An alternate approach to a simulcast system is disclosed in U.S. Pat. No. 4,188,582, assigned to the same assignee as the present invention. The system as therein disclosed utilizes a reference oscillator driving a microwave transmitter. The signal from the microwave transmitter is then conveyed to a plurality of microwave receivers which drive lower frequency transmitters which are phase-locked to the microwave receivers, thus providing a plurality of transmitters which are operating at relatively the same frequency. These systems operate within generally acceptable specifications, but the described system is quite complex and expensive.
Simulcast transmission systems used for paging devices typically transmit a combination of audio and data signals. A paging transmission can consist of a binary address and a voice message. In simulcast transmission systems which utilize phase-locked loops for deriving a reference frequency, a disparity of ones and zeros in the digital FSK signal will cause a net DC component in the received signal which will drive the included voltage-controlled oscillator of the phase-locked loop off frequency. This condition prevents effective data transmission with presently known phase-locked loop simulcast transmission techniques.
For the foregoing and other shortcomings and problems, there has been a long-felt need to reduce the complexity and expense of a simulcast transmission system while maintaining the transmitters at less than the allowable frequency difference while allowing transmission of both audio and data signals.